1. Field of the Invention
The invention relates to a resin tank weld member that allows a resin tank applied to, for example, a fuel tank or the like, to be attached with good precision and that restrains the escape of fuel, such as permeation of fuel and the like, from the fuel tank.
2. Description of the Related Art
In recent years, fuel tanks and, particularly, automotive fuel tanks, are subjected to increasingly severe requirements regarding the air-tightness for restraining the emission of fuel vapor that includes a hydrocarbon gas and the like to the outside, irrespectively of whether the vehicle is running or at a stop. It is strongly demanded as an urgent issue to reduce the leakage of fuel from the fuel tank as a matter of course, and also from various component parts attached directly or indirectly to the fuel tank, for example, a pipe fitting, a fuel control valve, a pump, a filter device, etc., and also sites of attachment to such component parts.
Furthermore, the demand for weight reduction and cost reduction of motor vehicles has also become severe. Along with this trend, fuel tanks made of plastics have been developed. At present, the use of tanks made of high-density polyethylene (HDPE) resin excellent in rust resistance and for weight reduction or the like has become mainstream. Accordingly, various component parts attached directly or indirectly to such a fuel tank are also made of resin. In order to facilitate the attachment of such component parts to the fuel tank, the same resin as that of the fuel tank is used for attachment portions of the component parts that are provided for attachment to the fuel tank, and means for joining the component parts and the fuel tank through heat welding is adopted.
In conjunction with the foregoing present circumstances, a fuel tank and various component parts attached to the fuel tank as described below are known.
That is, a fuel tank is formed integrally as one unit by using a resin in accordance with requirements for weight reduction and the like. To form such a fuel tank, a high-density polyethylene resin that is low in cost and excellent in impact resistance is used. However, the high-density polyethylene resin is known to allow the permeation of hydrocarbon although in small amount. To eliminate such a drawback, a laminated structure of a fuel tank wall as shown in FIG. 7 has been proposed.
A wall of the fuel tank 1 has a laminated structure that is basically composed of an inner shell 2 that constitutes an inside portion of the tank, an outer shell 3 that constitutes an outside portion of the tank, and a barrier layer 4 that is interposed between the inner shell 2 and the outer shell 3 and that prevents the permeation of hydrocarbon gas. In addition, adhesion layers are provided between the barrier layer 4 and the inner shell 2 as well as between the barrier layer 4 and the outer shell 3.
Concretely, the inner shell 2 and the outer shell 3 are formed by using a conventional high-density polyethylene resin, and the adhesion layers are formed by using a modified high-density polyethylene resin having adhesiveness (which, despite having adhesiveness, allows the permeation of hydrocarbon gas) which is obtained by improving a function of a high-density polyethylene resin, and the barrier layer 4 is formed by using a resin that prevents the permeation of hydrocarbon gas, for example, an EVOH resin (Eval) made by Kuraray, Co. Ltd., etc. Thus, countermeasures against the fuel permeation can be said to be substantially perfectly taken with respect to the fuel tank.
A pipe fitting 5 shown in FIG. 6 is known as a resin tank weld member that is attached to the fuel tank 1. The pipe fitting 5 has a body portion 6 and a tube portion 7. The body portion 6 is a cup-shape portion made of a high-density polyethylene resin, and has in its bottom portion a tubular weld portion 8. The weld portion 8 is welded to an outer peripheral surface of an upper portion of an opening 9 provided in an upper wall of the resin tank 1. Besides, the tube portion 7 is a cylindrically tubular polyamide resin-made portion that is formed on a side surface of an upper portion of the body portion 6 so as to be integral with the body portion 6. A hose is pressingly connected to a distal end of the tube portion 7, and links it in communication with a canister (not shown), so that fuel vapor from the fuel tank 1 is adsorbed to the canister.
A pipe wall of the pipe fitting 5 is formed by an inner peripheral layer 10 made of a polyamide (PA) resin of low permeability, and an outer peripheral layer 11 made of a modified high-density polyethylene resin excellent in the weldability to the fuel tank 1 and the adhesiveness to the polyamide resin of the inner peripheral layer 10. The inner peripheral layer 10 and the outer peripheral layer 11 are formed as follows. Firstly, the outer peripheral layer 11 is formed by pouring a modified high-density polyethylene resin into a mold. Then, the inner peripheral layer 10 is formed by pouring a polyamide (PA) resin to an inner side of the outer peripheral layer 11. That is, the pipe fitting 5 is produced by two-step forming (Japanese Patent Application Publication No. 2004-11419 (JP-A-2004-11419)).
The conventional polyamide (PA) resin of low permeability is able to prevent the emission of fuel gas into the atmosphere, but is not able to prevent it perfectly. For example, the polyamide (PA) resin is not altogether sufficient to meet the “P-ZEV” regulation that is already in force in the United States.
With regard to the conventional component, there are disclosed a method that uses a composite resin composition of polyolefin and polyamide containing an aromatic polymer (Japanese Patent Application Publication No. 2005-298639 (JP-A-2005-298639)), and a method that uses a composite resin composition of a polyamide resin and a polyphenylene sulfide resin (Japanese Patent Application Publication No. 2002-284991 (JP-A-2002-284991)). However, these methods have problems, for example: an expensive resin composition is used; there is constraint in the design of the mold since a band-like dispersion layer is formed in the molding; etc. Furthermore, there is also disclosed a method that uses a resin obtained by compounding a polyolefin resin and a lamellar silicate in a polyamide resin (Japanese Patent Application Publication No. 2001-302910 (JP-A-2001-302910)). This method is different from the invention in that the polyolefin layer in the method is a continuous layer. Therefore, this conventional method has problems of being poor in the heat-weldability to the fuel tank member despite being excellent in terms of the gas permeation.
As for the resin composition incorporating a lamellar silicate, there have been proposed a composition obtained by polymerizing 100 mass parts of polyamide in coexistence with an acid whose pKa is 0 to 6, and further mixing polyolefin or the like into the polymer (Japanese Patent No. 3409921), a resin composition obtained by mixing an ultrahigh molecular weight polyolefin and a resin in which a swellable synthetic fluorine mica is homogeneously dispersed in a polyamide resin (Japanese Patent Application Publication No. 10-279792 (JP-A-10-279792)), and a mixture of a polyolefin or the like and a resin composition in which a swellable fluorine mica-based mineral is dispersed at a molecular level in order to achieve high oxygen gas barrier characteristic (Japanese Patent Application Publication No. 2001-98147 (JP-A-2001-98147)). However, in each one of these compositions, the amino-terminal group concentration in the polyamide resin or the amount of maleic anhydride in the polyolefin and the like is not controlled to an amount that is suitable to the component part that is welded to the fuel tank. Therefore, with regard to the obtained mixture with polyolefin or the like, there is a problem in the compatibility between good welding strength to the polyolefin resin and good resistance to the gas permeation. It has been requested a measure to decrease a leakage of a fuel to the outside as much as possible.
In addition, in the conventional art, the polyamide resin of low permeability is provided only up to a vicinity of the weld portion 8 of the fuel tank 1, and fuel leakage occurs as stated below. That is, as shown in FIG. 7, fuel vapor shown by arrowed lines encircled one and encircled seven is shielded by the inner peripheral layer 10 and the barrier layer 4, respectively, so that substantially no fuel vapor is emitted to the outside.
However, as for a flow of fuel vapor as shown by an arrowed line encircled three, fuel vapor is emitted to the outside through the outer peripheral layer 11 that is not of low permeability as shown by an arrowed line encircled five, and is also emitted to the outside through the outer shell 3 that is also not of low permeability as shown by an arrowed line encircled six.
Furthermore, in the foregoing conventional art, since the two-step forming in which the inner peripheral layer 10 is poured to the inner side of the outer peripheral layer 11 is adopted, there is a drawback of an increase in the number of formation processes and therefore a corresponding rise in the production cost. Furthermore, the two-step forming allows the formation of shapes only to a limited range, and thus suffers from a drawback of a constraint on the degree of freedom in the shape.